Remove `G` generic param from `Diag<'_, G>` by nnethercote · Pull Request #163083 · rust-lang/rust

nnethercote · 2026-09-20T23:59:42Z

The benefits are outweighed by the costs. Details in invidual commits.

some minor earlier discussion about this change in #162630 (comment)

It encodes the `emit` behaviour, one of: - `BugAbort`: Abort as a bug. - `FatalAbort`: Abort as a fatal error. - `ErrorGuaranteed`: Return an `ErrorGuaranteed` (the default). - `()`: return `()`. This is useful but `Diag` is very widely used and it's arguably not useful enough to be worth the generic parameter. This commit removes it. Benefits: - Many fewer generic parameters, including many functions where the genericity is entirely uninteresting. This includes every `Diagnostic::into_diag` and `Subdiagnostic::add_to_diag`. Removes the need for the comment about `<G>` on `trait Diagnostic`. - No `PhantomData` in `Diag`. - Makes bootstrapping a bit faster. The downside is there is no longer a single `emit` method. - Getting a `!` return type requires calling `emit_bug` or `emit_fatal`. This only affects a few call sites. - Getting an `ErrorGuaranteed` return type requires calling `emit_err`. This affects a lot of call sites. (It arguably makes call sites more self-documenting.) - Using the wrong `emit_*` on a diagnostic causes a runtime abort, which previously couldn't occur. Seems unlikely, but it's not impossible. - Note: If no particular return type is needed, `emit` can be used for any diagnostic. (Bug/fatal diagnostics will still trigger abort.)

rustbot · 2026-09-20T23:59:45Z

The rustc-dev-guide subtree was changed. If your future PRs only touch the subtree, consider submitting them directly to rust-lang/rustc-dev-guide, which is where the document is primarily maintained (and has faster CI).

cc @BoxyUwU, @tshepang

clippy is developed in its own repository. If possible, consider making this change to rust-lang/rust-clippy instead.

cc @rust-lang/clippy

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann, @JonathanBrouwer

miri is developed in its own repository. If the Miri part of this change can be broken out, consider making this change to rust-lang/miri instead. However, if Miri needs adjusting for rustc changes, just ignore this message.

cc @rust-lang/miri

The parser was modified, potentially altering the grammar of (stable) Rust
which would be a breaking change.

cc @fmease

Some changes occurred in match checking

cc @Nadrieril

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

Some changes occurred to constck

cc @fee1-dead

HIR ty lowering was modified

cc @fmease

rustc_macros::diagnostics was changed

cc @TaKO8Ki, @JonathanBrouwer

Some changes occurred in const_evaluatable.rs

cc @BoxyUwU

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

nnethercote · 2026-09-20T23:59:51Z

@bors try @rust-timer queue

Remove `G` generic param from `Diag<'_, G>`

rust-bors · 2026-09-21T01:01:36Z

☀️ Try build successful (CI)
Build commit: 282876a (282876a4efd9e5ce0c9e41d5663a524acdc572a0)
Base parent: bba5310 (bba531001d4de6d7f49693e0836a2668ca063282)

rust-timer · 2026-09-21T01:46:10Z

Finished benchmarking commit (282876a): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@bors rollup=never rustc-perf
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.3%	[0.1%, 0.4%]	4
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.4%	[-0.8%, -0.3%]	8
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary 2.2%, secondary -3.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.2%	[2.2%, 2.2%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.0%	[-3.0%, -3.0%]	1
All ❌✅ (primary)	2.2%	[2.2%, 2.2%]	1

Cycles

Results (primary 3.2%, secondary 1.9%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	3.2%	[3.2%, 3.2%]	1
Regressions ❌ (secondary)	3.9%	[2.7%, 5.0%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.0%	[-2.0%, -2.0%]	1
All ❌✅ (primary)	3.2%	[3.2%, 3.2%]	1

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 496.585s -> 488.23s (-1.68%)
Artifact size: 408.87 MiB -> 406.49 MiB (-0.58%)

oli-obk

View changes since this review

rust-bors · 2026-09-21T10:52:46Z

📌 Commit 30b8dfc has been approved by oli-obk

It is now in the queue for this repository.

JonathanBrouwer · 2026-09-21T11:05:54Z

@bors rollup=iffy
Perf changes are small enough

Remove `G` generic param from `Diag<'_, G>` The benefits are outweighed by the costs. Details in invidual commits. some minor earlier discussion about this change in rust-lang#162630 (comment)

…uwer Rollup of 6 pull requests Successful merges: - #163083 (Remove `G` generic param from `Diag<'_, G>`) - #160679 (Staticlib rename internal symbols: add COFF support) - #162007 (Preserve parentheses in raw borrow suggestions) - #163057 (Use span context for enclosing item(s) of unmet bound) - #163070 (label the path segment that is not an associated item of the trait) - #163078 ( Add a README in tests/rustdoc-html to explain what each folder content is for)

rust-bors · 2026-09-21T13:14:42Z

⌛ Testing commit 30b8dfc with merge 929d979...

Workflow: https://github.com/rust-lang/rust/actions/runs/35604359904

Remove `G` generic param from `Diag<'_, G>` The benefits are outweighed by the costs. Details in invidual commits. some minor earlier discussion about this change in #162630 (comment)

JonathanBrouwer · 2026-09-21T13:19:45Z

@bors yield
Yielding to enclosing rollup

rust-bors · 2026-09-21T13:19:51Z

Auto build was cancelled. Cancelled workflows:

https://github.com/rust-lang/rust/actions/runs/35604359904

The next pull request likely to be tested is #163111.

…uwer Rollup of 10 pull requests Successful merges: - #163083 (Remove `G` generic param from `Diag<'_, G>`) - #160679 (Staticlib rename internal symbols: add COFF support) - #160859 (`core::num::f16b` Rust's 16bit Brain Float) - #162007 (Preserve parentheses in raw borrow suggestions) - #162821 (Avoid suggesting closures when captures cross associated item boundaries) - #163057 (Use span context for enclosing item(s) of unmet bound) - #163070 (label the path segment that is not an associated item of the trait) - #163078 ( Add a README in tests/rustdoc-html to explain what each folder content is for) - #163107 (Implement Allocator for Pin unstably) - #163108 (remove old solver use of `SolverRelating`)

rust-bors · 2026-09-21T13:26:57Z

⌛ Testing commit 30b8dfc with merge e4f44de...

Workflow: https://github.com/rust-lang/rust/actions/runs/35605678728

Remove `G` generic param from `Diag<'_, G>` The benefits are outweighed by the costs. Details in invidual commits. some minor earlier discussion about this change in #162630 (comment)

JonathanBrouwer · 2026-09-21T13:30:49Z

@bors yield
Yielding to enclosing rollup

rust-bors · 2026-09-21T13:30:53Z

Auto build was cancelled. Cancelled workflows:

https://github.com/rust-lang/rust/actions/runs/35605678728

The next pull request likely to be tested is #163111.

…uwer Rollup of 10 pull requests Successful merges: - #163083 (Remove `G` generic param from `Diag<'_, G>`) - #160679 (Staticlib rename internal symbols: add COFF support) - #160859 (`core::num::f16b` Rust's 16bit Brain Float) - #162007 (Preserve parentheses in raw borrow suggestions) - #162821 (Avoid suggesting closures when captures cross associated item boundaries) - #163057 (Use span context for enclosing item(s) of unmet bound) - #163070 (label the path segment that is not an associated item of the trait) - #163078 ( Add a README in tests/rustdoc-html to explain what each folder content is for) - #163107 (Implement Allocator for Pin unstably) - #163108 (remove old solver use of `SolverRelating`)

Rollup merge of #163083 - nnethercote:rm-Diag-G, r=oli-obk Remove `G` generic param from `Diag<'_, G>` The benefits are outweighed by the costs. Details in invidual commits. some minor earlier discussion about this change in #162630 (comment)

…uwer Rollup of 10 pull requests Successful merges: - rust-lang/rust#163083 (Remove `G` generic param from `Diag<'_, G>`) - rust-lang/rust#160679 (Staticlib rename internal symbols: add COFF support) - rust-lang/rust#160859 (`core::num::f16b` Rust's 16bit Brain Float) - rust-lang/rust#162007 (Preserve parentheses in raw borrow suggestions) - rust-lang/rust#162821 (Avoid suggesting closures when captures cross associated item boundaries) - rust-lang/rust#163057 (Use span context for enclosing item(s) of unmet bound) - rust-lang/rust#163070 (label the path segment that is not an associated item of the trait) - rust-lang/rust#163078 ( Add a README in tests/rustdoc-html to explain what each folder content is for) - rust-lang/rust#163107 (Implement Allocator for Pin unstably) - rust-lang/rust#163108 (remove old solver use of `SolverRelating`)

nnethercote added 2 commits September 21, 2026 09:57

Convert create_err(e).emit() occurrences to emit_err(e)

23ba1fa

rustbot assigned oli-obk Sep 20, 2026

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rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Sep 20, 2026

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rust-bors Bot pushed a commit that referenced this pull request Sep 20, 2026

Auto merge of #163083 - nnethercote:rm-Diag-G, r=<try>

282876a

Remove `G` generic param from `Diag<'_, G>`

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rustbot added perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Sep 21, 2026

oli-obk approved these changes Sep 21, 2026

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rust-bors Bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Sep 21, 2026

JonathanBrouwer mentioned this pull request Sep 21, 2026

Rollup of 6 pull requests #163105

Closed

JonathanBrouwer mentioned this pull request Sep 21, 2026

Rollup of 10 pull requests #163111

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rust-bors Bot merged commit a7be294 into rust-lang:main Sep 21, 2026
14 of 15 checks passed

rustbot added this to the 1.100.0 milestone Sep 21, 2026

nnethercote deleted the rm-Diag-G branch September 21, 2026 21:48

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rust-timer commented Sep 21, 2026

Overall result: ❌✅ regressions and improvements - please read:

Instruction count

Max RSS (memory usage)

Cycles

Binary size

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